The invention relates to a low-pass filter for a diplexer to separate low-frequency signals of analog communications systems from high-frequency signals of digital communications systems with a plurality of longitudinal inductances connected in series and formed by means of magnetic cores.
The magnetic cores heretofore used were RM4, RM6, RM8 as well as other ferrite shell cores made of core materials such as, for example, N27 and N48. The required direct field bias capability and the necessary linearity of the hysteresis loop were achieved by means of shear by slitting the ferrite core.
A drawback of shear is that it causes the effective core permeability to be decreased to values of about 200. To reach the required magnetizing inductance, the volume of the ferrite core must therefore be very large, so that a low-pass filter made of ferrite cores requires a lot of space. A further disadvantage of the ferrite cores is the high number of turns of the primary and secondary windings, which may cause ohmic losses and capacitive interference effects.
EP 0 677 938 describes a diplexer comprising a low-pass filter and a high-pass filter. The low-pass filter has longitudinal inductances that are connected in series and are formed by means of ferromagnetic cores.
EP 0 780 854 describes a current-compensated radio interference suppression choke that has a magnetic core made of a nanocrystalline alloy. The magnetic core has a permeability ranging from 10,000 to 60,000, a saturation induction greater than 1 Tesla and a specific resistance greater than 90 xcexcxcexa9. Based on the high permeability and the high saturation induction, the current-compensated radio interference suppression choke has a particularly small configuration. The properties of the nanocrystalline alloy are such that good interference suppression is achieved despite the small configuration of the radio interference suppression choke.
Based on this prior art, it is the object of the invention to create a low-pass filter with low unit volume that meets the requirements for application in a diplexer with respect to direct field bias capability and filter quality.
This object is attained according to the invention in that the magnetic cores are made of an alloy and the harmonics of the low-frequency signals can be suppressed by the impedance of the longitudinal inductances, which increase with increasing frequency.
Amorphous and nanocrystalline alloys make it possible to produce magnetic cores with high saturation induction and a broad range of permeability values. Compared to conventional ferrite cores, magnetic cores made of an amorphous or nanocrystalline alloy have therefore a substantially smaller unit volume with comparable magnetic properties. It is difficult, however, to produce magnetic cores of amorphous or nanocrystalline alloys in such a way that the hysteresis loop has the linearity required for application in broadband communications systems. A high measure of linearity is required, however, to ensure constant permeability and thus constant inductance of the low-pass filter chokes. Otherwise dimensioning of the low-pass filter chokes is made substantially more difficult. It is generally required, however, to connect several inductances in series to obtain the required filter quality. This series connection of the longitudinal inductances increases the interference attributable to non-linearities of the hysteresis loop. The harmonics produced by non-linearities of the hysteresis loops are attenuated, however, by the impedance of the longitudinal inductances, which increases with increasing frequency. It is thus possible, despite the lack of linearity of the hysteresis loops of the employed magnetic cores, to build a low-pass filter from an amorphous or nanocrystalline alloy, which satisfies the linearity requirements of the transfer characteristic. Due to the higher saturation induction and permeability of the nanocrystalline and amorphous materials compared to ferrites, smaller configurations for the magnetic core furthermore result.
The preferred alloys for use in low-pass filters are the subject of the dependent claims.